Intra-annual Variation of Eddy Diffusion (k$_{zz}$) in the MLT,
from SABER and SCIAMACHY Atomic Oxygen Climatologies
Abstract
Atomic oxygen (O) in the MLT (mesosphere and lower thermosphere) results
from a balance between production via photo-dissociation in the lower
thermosphere and chemical loss by recombination in the upper mesosphere.
The transport of O downward from the lower thermosphere into the
mesosphere is preferentially driven by the eddy diffusion process that
results from dissipating gravity waves and instabilities. The motivation
here is to probe the intra-annual variability of the eddy diffusion
coefficient (k$_{zz}$) and eddy velocity in the MLT based on the
climatology of the region, initially accomplished by
\citeA{GarciaandSolomon1985a}. In the current study,
the intra-annual cycle was divided into 26 two-week periods for each of
three zones: the northern hemisphere (NH), southern hemisphere (SH), and
equatorial (EQ). Sixteen years of SABER (2002-2018) and 10 years of
SCIAMACHY (2002-2012) O density measurements, along with
NRLMSIS\textsuperscript{\textregistered}
2.0 were used for calculation of atomic oxygen eddy diffusion velocities
and fluxes. Our prominent findings include a dominant annual oscillation
below 87 km in the NH and SH zones, with a factor of 3-4 variation
between winter and summer at 83 km, and a dominant semiannual
oscillation at all altitudes in the EQ zone. The measured global average
k$_{zz}$ at 96 km lacks the intra-annual variability of upper
atmosphere density data deduced by \citeA{Qian2009}.
The very large seasonal (and hemispherical) variations in k$_{zz}$
and O densities are important to separate and isolate in satellite
analysis and to incorporate in MLT models.